Rotary couplings are known, such as from GB 2343923 which utilises coupling members carried by a driving part and seated in recesses on a driven part. A driving torque can thus be transmitted from the driving part to the driven part through the coupling members, but the application of a reverse torque, i.e. where a torque is applied externally to the driven part of the coupling causing the driven part to attempt to drive the driving part, such as in a runaway condition, causes the coupling members to move or tilt relative to their recesses, thereby engaging a fixed braking surface on the rotary coupling and locking it up.
However, there are times when it is necessary to be able to allow transmission of a reverse torque. For example, it is a requirement that motorised wheel-chairs be capable of having a free-wheel state, i.e. where the application of a reverse torque is permitted without locking the rotary coupling, as well as a motor-driven state. Obviously, use of the above-mentioned known rotary coupling prevents the wheel-chair ever assuming a dangerous runaway condition, but also fails to provide for the required free-wheel state.
A further problem is apparent in battery operated drive systems, for example in stairlifts, where it is a requirement to have a battery-operated emergency back-up system, and in electric vehicles, such as golf carts. These drive systems commonly utilise worm-drive gearbox arrangements due to their inherent ability to prevent or inhibit the transmission of reverse torque (i.e. back-driving). However, it is well-known that worn-drive gearbox arrangements are extremely inefficient, generally in the order of 70%. As a consequence, bigger or more battery packs are required to enable longer running times, which itself leads to increased weight and reduced space problems.
It is known that reduction-type gearbox arrangements are far more efficient than worm-drive gearbox arrangements. However, reduction gearbox arrangements have a much reduced ability to inhibit back-driving, and are thus generally not preferred.
Yet another problem is apparent in clutch arrangements. Friction-plate clutch arrangements are in common-use, but the plates tend to wear causing unreliable torque transmission.
The present invention seeks to overcome these problems.